Implantable medical devices come in a variety of configurations. Devices such as pacemakers, implantable cardioverter/defibrillators (ICD) or cardiac stimulation devices incorporating both the functionality of pacemakers and ICDs are commonly used devices that provide stimulation therapy to a patient in order to regulate heart function. Implantable cardiac stimulation devices can generally apply a variety of different electrical waveforms to the patient in order to treat various different arrhythmias that affect heart function. Preferably, at implantation, it is desirable to be able to test the implantable medical device to ensure that the implantable medical device will deliver appropriate therapy to the patient.
Hence, at implantation various testing procedures have been implemented with implantable medical devices to assess their ability to deliver appropriate therapy to the patient. For example, at implantation of an ICD, it is common for the physician to induce fibrillation of the heart to ensure that the ICD will detect the fibrillation and will also apply the appropriate stimulation to the heart to terminate the fibrillation. Naturally this procedure is not without some risk as the ICD may either be unable to detect or terminate the fibrillation. At that point, an external defibrillator is typically used in order to termination the fibrillation.
Due to the risks associated with testing implantable cardiac stimulation devices, there has been a need for systems that test the implantable cardiac stimulation device without posing substantial risk to the patient. Some examples of external testing devices are disclosed in U.S. Pat. No. 5,237,991 to Baker Jr. et al. and also in U.S. Pat. No. 5,314,452 to Hershberg et al. These types of devices typically include a fixed resistive load that is positioned across the output terminals of the cardiac stimulation device. The device can then be induced to produce a cardiac stimulation waveform and the results of the waveform can then be assessed typically using delimited data that is transmitted to an external programmer.
While these devices reduce the risk associated with having the implantable cardiac stimulation device attempt to terminate a particular arrhythmia at implantation, these devices have several shortcomings. Initially, these devices typically only use a single fixed load which is often not representative of the actual load of the heart when therapy is actually being applied. This is particularly problematic for implantable cardiac stimulation devices which have a capability of applying a variety of different defibrillation, cardioversion or pacing waveforms to different regions of the heart to correct various arrhythmias. Moreover, simply positioning a fixed load across the output terminals of an implantable cardiac stimulation device only tests the ability of the device to deliver a waveform but does not provide any indication as to the ability of the implantable device to detect various arrhythmias. Consequently, these types of devices generally do not reduce the need to test the device on the actual patient with all of the inherent risks.
In some circumstances, many physicians may be unwilling to test the device on the patient on implantation due to the inherent risks. For example, advanced pacing devices may also include high voltage capability that can provide either cardioversion or defibrillation waveforms to the heart. If the patient has never exhibited previous symptoms that would require the delivery of high-voltage waveforms, the treating physician may be unwilling to test the ability of the device to deliver these waveforms to the patient due to the risk. Consequently, if the patient does subsequently exhibit an arrhythmia that requires a high voltage waveform, the device may be unable to provide the waveform and this inability of the device would have gone undetected.
From the foregoing, it should be appreciated that there is a need for a system that is capable of assessing the ability of an implantable medical device to detect circumstance that require the delivery of therapeutic stimulation to a patient without requiring the device to be tested on the patient itself. To this end, there is a need for a device which more accurately simulates the patient and can also be configured to test not only the ability of the implantable medical device to deliver therapeutic waveforms, but also the ability of the device to sense the existence of a circumstance which would require the delivery of the therapeutic waveforms.